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Running core barrels through whipstocks
23 October 2013
At a recent DWOP, we were discussing the plan to cut 30 ft of  8"" core in a sidetracked section.

The initial bore is vertical 'FINDER well type' with minimal data acquisition planned.  In the 'success case', the well will be sidetracked to acquire a 30' core and perform a DST.

The whipstock (set in 9⅝") will result in 6°/100 ft dogleg severity (DLS). The coring assembly comprises 8"" corehead, 6¾" barrel.  The Coring Co advised that 3°/100 ft DLS is the maximum recommended.

We discussed options which include:
  1. Section-milling a longer window (instead of the whip-stock) and kicking off a cement plug set across the window, but there's no guarantee that the DLS will be any less.
  2. Setting whipstock as originally planned and drilling 8"" hole to core point and then running a smaller (6") coring assembly, which will easily pass through the window.
It would be good to hear from those of you have faced and overcome this challenge.

Thanks and kind regards

3 answer(s)
Lead Petrophysicist
Maersk Oil
Total Posts: 2
Join Date: 24/02/11
There are three key operational phases that pose technical challenges :
1) RIH without compromising wellbore and drillstring (including core barrel)
2) Rotating during coring ops
3) POOH and not damaging core.

In the debate so far, core damage is under-emphasised. By bending core, it can be severely mechanically damaged hence significantly degrading its value. Thus, low dog leg severity is a must.
Global Technical Director - Coring Services
National Oilwell Varco
Total Posts: 5
Join Date: 01/07/12
Hi Dave,
Although on paper most high torque core barrels (double shoulder) for 8-1/2" hole would be able to meet & exceed 6 deg / 30m DLS (static - but most likely not for rotating)- in practice it will be very tight & likely an issue.

Most coring service providers have stabiliser spacing 30ft (9m), but there is one vendor with 20ft (6m) stabiliser spacing, as you can imagine this will effect DLS capability.

Without specifics on the well environment (formation properties, final inclination & other conditions) another option may be to replace the stabilisers with "slick subs"; if it's only 30ft you need this may be the answer.
Check with your vendor, they may also be able to pull in an 8-3/8" OD Corehead from other areas to assist (Middle East for example) in negotiating going across the whipstock. 

Running a rathole assembly for 6" hole as you mention is also a good plan for 30ft - subject to other factors.
Drilling Supt
Nostrum Oil & Gas
Total Posts: 45
Join Date: 14/11/10


there are two different OOS conditions for the DLS. There is a difference if you rotate or not => rotating bending moments or static bending moments. Just take a paper clip and apply static bending - then move your hands up and down with that bending and apply rotating bending - it will twist off very soon. 

So you should have got two different limits from the coring service.
8½ HT 30 coring BHAs from Baker Hughes have the following limits:  sliding DLS 4.2°/10m and 1.7°/10m rotating. 
Yes given per 10m  the TDS !  So 12.6°/30m sliding and 5.1°/30m rotating.  You can order the coring service fro Baker Hughes :-)

Apart from that also the rotating limitation on the thread of DCs (and Jar as well) do not allow to rotate through 6°/30m on the whipstock. So in case you need to drill out across the whipstock pick up an RSS with motor above and do not turn the string from surface and as well use HWDPs instead of DCs in case you need to rotate the string from surface across the whipstock.

The 6°/30m on the whipstock sounds a bit low as well - go and check the drawing for the whip stock and I bet (a box of beer) that it is more than 6°/30m when you take inclination out and ramp start and end (not whipstock length please). There are very short Sii whipstocks around with even 30°/30m DLS. 

In case you need to core just below the whipstock I would order a different whip stock. Using a motor above coring BHA is too much weight below bearing of motor and would demand too much TQ as well.

How long is the rat hole planned to be drilled with the milling BHA from the whipstock ?

Making the coring BHA less stiff and leave out the fullgauges stabilzers all 9m could help as well (but BHA will not hold inclination then) and rotating with minimum RPM from surface to reduce bending moments.

Calculation of bending moments across BHA is part of the standrad procedures in Baker Hughes (INTEQ).


Peter-Jørn Palten
working in the moment in Stavanger Statoil office for East Africa deep water exploration.

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